1. Field of the Invention
The present invention relates to a process for preparing a granular polyarylene sulfide having a high molecular weight and a controlled particle size and, more particularly, to a process for preparing a granular polyarylene sulfide by using a polymerization aid so as to adjust a polymerization temperature, the granular polyarylene sulfide being suitable for the preparation of sheets, films, fibers and so on, causing less burrs to occur upon injection molding, having such a sufficiently high molecular weight as providing the property of forming a gel, and being easy to handle.
2. Description of Related Art
Among polyarylene sulfides such as polyphenylene sulfides, some are thermoplastic resins having partially thermosetting characteristics, which have excellent properties as engineering plastics, such as resistance to chemicals, good mechanical properties over a wide range of temperatures, and resistance to thermal rigidity.
Recently, demands have been made on processes for preparing polyarylene sulfides such as polyphenylene sulfides, which do not make them highly viscous by heating, which can provide them with a sufficiently high molecular weight only by polymerization, and which cannot impart color to them.
Heretofore, there have been made various proposals for such demands. For example, Japanese Patent Laid-open (kokai) Publication No. 46,228/1988 discloses a process for preparing a polyarylene sulfide by reacting a dihalogen aromatic compound with an alkali metal sulfide in a polar solvent, which comprises an early-stage polymerization step for forming a prepolymer having a low melting viscosity and a later-stage polymerization step for proceeding with further polymerization of the resulting prepolymer, wherein the early-stage polymerization step is carried out by reacting the dihalogen aromatic compound in such a state in which the arylene group and water are present at the rates of 0.5 mole to 3.5 moles and 0.5 mole to 5 moles, respectively, with respect to kilogram of an organic solvent in the reaction system, thereby converting the dihalogen aromatic compound at the conversion rate of 50% to 98% by mole into the prepolymer having a melting viscosity of 5 poises to 300 poises, and the later-stage polymerization step is carried out without separation of the resulting prepolymer from the reaction system by adding water to the reaction system so as to allow the water content in the reaction system to account for 1.5 mole to 15 moles per kilogram of the organic solvent, retaining the resulting reaction system at temperatures ranging from 257.degree. C. to 290.degree. C. with stirring for at least 10 minutes and then stirring the reaction system for at least 2 hours at temperatures reduced to the range from approximately 220.degree. C. to 250.degree. C. This polymerization process can yield the polyarylene sulfide with a high molecular weight, which is easy to handle.
Further, for example, Japanese Patent Laid-open (kokai) Publication No. 9,266/1989 discloses a polyarylene sulfide with improved burr characteristics, which is prepared by blending a non-crosslinked polyarylene thioether having a melting viscosity of 10 poises to 5.times.10.sup.4 poises and a crosslinked polyarylene thioether having a melting viscosity of 5.times.10.sup.5 poises to 1.times.10.sup.9 poises with a fibrous filler and/or inorganic filler.
It cannot be said, however, that the process disclosed in Japanese Patent Laid-open (kokai) Publication No. 46,228/1988 comprises at least three steps for polymerization, including the step for preparing the prepolymer and it requires a long reaction period of time. And the polyarylene sulfide obtainable by this process has a relatively large molecular weight yet its molecular weight is not so high as demonstrating the property of forming a gel.
It is also noted that some of the polyarylene sulfides obtainable by the process disclosed in Japanese Patent Laid-open (kokai) Publication No. 9,266/1984 contain granules at the rate of approximately 20%, yet a majority of them are in a bulky state. Therefore, they are not said to be desired in handling.
As an improvement of this process, there has been proposed the process as disclosed in Japanese Patent Laid-open (kokai) Publication No. 299,826/1989. This process is a process for preparing a crosslinked polyarylene sulfide in a granular form, which comprises three steps including a step for converting a halogenated aromatic compound at the conversion rate of 50% to 98% by mole by using a branching agent in the presence of water in a predetermined amount at temperatures below 180.degree. C. to 235.degree. C., a step for elevating the temperature of the reaction system under a particular condition so as to yield a polymer having a particular range of its melting viscosity in the presence of a particular amount of water, and a step for continuing the reaction at temperature ranging from 245.degree. C. to 290.degree. C.
It is to be noted, however, that this process presents the problems that the steps and setting of reaction conditions are complex and the polymerization temperature in the first step is so low that the polymerization period of time becomes very long.